We studied a case of familial ataxia-telangiectasia in a 15-year-old girl who had a clinical history of cerebellar ataxia and recurrent pulmonary infections. She was found at autopsy to have a hepatocellular carcinoma, which has been described twice previously in the literature as occurring with this disorder. Family studies on the majority of her seven siblings (the product of one father and two mothers who were identical twins) showed one brother to have classic features of cerebellar ataxia, IgA deficiency, markedly elevated alpha-fetoprotein levels, and characteristic chromosomal abnormalities. This boy also died later of hepatocellular carcinoma in 1984. An affected sister had previously died of a respiratory tract infection.